This particular rule seems fairly asinine, honestly. Why not format this as a chapter titled “Introduction”? (APA also has stupid, different, rules about introductions.) Thinking carefully about this one, it seems too fiddly to bother with–it would require a to term as well as the empty-title, genre data entry structure.
Beyond this case, I support adopting genre and medium more generally in styles as a substitute for missing title. That’s the most reasonable approach to avoiding style-specific markup being entered into the title field (e.g., for the [Photograph of William James] example above). Like a lot of CSL best practices, I think these sorts of conventions can be reasonably incorporated into the major style guide styles (e.g., APA, Chicago, MLA, Vancouver), then gradually into more specific styles based on user demand.